Polyurethans are obtained by reacting long chain polyols, having a molecular weight from 400 to 10,000, with organic polyisocyanates, preferably diisocyanates, in the presence of chain extending agents generally consisting of glycols, polyols or amines having short chain and molecular weight lower than 400. Polyurethans are exploited for the manufacture of a wide variety of materials; in particular, elastomeric thermoplastic polyurethans can be processed by injection molding extrusion, calendering etc., to produce shaped articles useful in the field of industry. The use of such products to prepare materials showing a structural resistance equal to the one of the products known as "technopolymers" (for instance nylon) is however limited by the necessity of using reinforcing materials, such as glass or carbon fibres, in order to obtain the desired rigidity, impact resistance and so on.
The features of polyurethans, in particular hardness, can be changed within a wide range of values comprised between those of the very soft materials and those of hard materials having a limited flexibility, by changing the amount of the various components used for their preparation: polyols, polyisocyanates and chain extending agents.
In particular, this change of features can be reached by changing, the --NCO group being the same, the weight ratio between the long chain polyol and the short chain polyol. In fact, the units of the polymeric chain derived from the low molecular weight extending agent and from the polyisocyanate give rise to hard segments having a high elastic modulus, whereas the chain units deriving from the high molecular weight polyol give rise to soft segments, having a low elastic modulus, due to the presence of the long chain of the polyol. In the case of an essentially linear polyurethan (prepared from a diisocyanate, a difunctional polyol and a difunctional extending agent), an increase of the percentage of the extending agent (in comparison with the polyol) is likely to embody into the chain a higher percentage of hard segments and this makes the polyurethans more and more hard, but more brittle; namely the material, although thermoplastic, loses its elastomeric properties. The no more elastomeric material shows, due to the high percentage of hard segments, a hardness higher than 70 Shore D and a high flexural modulus.
It is known from U.S. Pat. No. 3,356,650 to prepare a thermoplastic polyurethan showing a hardness higher than 70 Shore D, using high ratios between hard and flexible segments. According to the process described in such patent, thermoplastic polyurethans are prepared by reacting the organic polyisocyanate with a mixture of reagents consisting of at least one compound containing three active hydrogen atoms and other organic compounds containing two active hydrogen atoms, determined according to the Zerewitinoff method, said active hydrogen atoms being reactive with the isocyanate group. The main characteristics of the process described in said U.S. patent are as follows:
(1) The reactants mixture must have an average molecular weight lower than 500, whereas each single component of the mixture can have a higher molecular weight, up to 5,000 or higher;
(2) the mixture must contain an amount of components having a functionality (for each molecule) higher than 2, in such a manner that from 0,01 to 20% of the groups containing the active hydrogen atoms are supplied by the compounds containing more than 2 functional groups per molecule; some of the properties, such as thermoplasticity, is negatively affected by the values out of these limits;
(3) the reactants are used in such an amount as to keep the equivalent NCO/OH ratio between 0.7 and 1.3.
The conditions above are crytical to produce hard polyurethans having high heat distorsion temperature and high hardness (even higher than 80 Shore D). However, when particular polyisocyanates and/or diols and/or particular process conditions are used, the polyurethans can be brittle and therefore not suitable for application in those fields where engineering type services are requested.
The Applicant has now succeeded in overcoming these drawbacks and therefore in producing polyurethans characterized by high impact-resistance, high flexural modulus and high heat distortion temperature, by carrying out the reaction between the organic polyisocyanate, the long chain polyol and the chain extending agent under particular process conditions.